The present invention relates to a method and to an apparatus for maintaining or increasing the temperature of a metal melt present in a receiving vessel after discharge from a melting furnace.
Temperature losses occur in a melt when charges are taken from the melting furnace, during metallurgical post-treatments, during ladle transport and during standing and casting of the melt, particularly into the tundish of a continuous caster. Metallurgical posttreatment in connection with steel melts may be, for example, alloying, washing with inert gas, blowing in desulphuring agents and vacuum degasification of the melt for the purpose of setting the desired composition, deoxidation and separation of the deoxidation products, homogenization, desulphuring and reduction of the oxygen and nitrogen contents of the metal as well as controlled setting of the casting temperature. Such post-treatments may be effected, for example, in the casting ladle, in a vacuum treatment vessel or in a tundish. The receiving vessel in the sense of the present invention thus also includes vessels through which the melt flows while forming an accumulation level.
It is possible to compensate for the above-described temperature losses by correspondingly overheating the melt in the melting furnace. But then it must be accepted that the furnace lining will be stressed more by the higher temperature and the production rate of the melting furnace will be cut down for the time required for the overheating.
It is known to return the thermal energy lost from the melt by the above-described processes by means of a heating device disposed outside the melting furnace. Such heating devices, which generally are disposed in the casting ladle, are electric arc heaters operating with three graphite electrodes each and with three-phase current. To quickly move the graphite electrodes up and down, they are equipped with a complicated lifting structure and with an electrode regulating device for each one of the heavy graphite electrodes.
Due to the diameter of the electrodes, and the dimensions of the current supplies and of the electrode mounts, the space between the electrodes is relatively large and their spacing from the ladle wall is correspondingly small. Moreover, electromagnetic forces cause the three-phase current arcs of the graphite electrodes, which are not very stable in any case, to burn in the direction toward the ladle wall so that the latter is under great thermal stress and wear of the ladle lining is correspondingly high. To overcome these drawbacks, the graphite electrodes are operated with the shortest possible arcs but this increases the danger of recarbonization of the melt from the electrode graphite.
The known heating device has the further drawback that the passages in the ladle cover can be made gastight against the hot graphite electrodes only with complicated structural measures so that the penetration of air damaging to the metallurgical treatment must be prevented by cost-intensive blowing in of pressurized inert gas.
To avoid premature wear of the walls, it is known to immerse an electrode arrangement composed of two graphite electrodes into the metal melt with the inner, rod-shaped electrode being offset to the rear with respect to the outer, tubular electrode. However, this known heating device has the inherent danger of causing the metal melt to carbonize. Additionally, the outer, tubular electrode is subject to great wear.
Inductive heating of metal melts is also known. This, however, requires equipping all receiving vessels to be used with induction coils. Moreover, this treatment also involves a not insignificant wear of the ladle lining.